Burnish resistant powder coating composition

ABSTRACT

The present invention provides a low gloss powder coating composition comprising a polyester having hydroxyl and carboxyl groups and glycidyl-containing acrylic copolymer, the copolymer having a T g  less than about 55° C. and a number average molecular weight greater than about 8000.

This application is a divisional of U.S. pat. application Ser.No.09/386,882, filed Aug. 31, 1999, now U.S. Pat. No. 6,310,139, thedisclosure of which is incorporated by reference herein in its entirety.

FIELD AND BACKGROUND OF THE INVENTION

This invention relates to a powder coating composition, and moreparticularly a powder coating composition that exhibits excellentresistance to degradation and is burnish resistant.

Powder coating compositions are now being used in a wide variety ofpainting techniques. They have become increasingly desirable,particularly in the automotive and track industries, for various reasonsincluding those related to ecology, health and safety. For example,powder coating compositions can reduce the amount of volatile solventsused as compared to liquid paints. After curing, typically using heat,only a very small amount, if any, of volatile solvent is given off tothe environment.

Various powder coating compositions have been suggested. For example, apowder coating composition with excellent exterior durability can beprepared by reacting an acid group-containing acrylic polymer and acuring agent, triglycidyl isocyanurate (TGIC). Such coatingcompositions, however, often have poor stability and physicalproperties.

U.S. Pat. No. 4,499,239 to Murakami et al. proposes a compositioncomprising 60 to 97 percent by weight of a linear polyester resin havingan acid number of 15 to 200 mg KOH/g and 3 to 40 percent by weight of aglycidyl group-containing acrylic polymer, and optionally is modifiedwith a vinyl monomer such as methyl methacrylate. Powder coatingcompositions comprising a copolymer of glycidyl methacrylate, anethylenically unsaturated compound, and a crosslinking agent formed inan anhydride of a dicarboxylic acid are proposed in U.S. Pat. Nos.3,758,632, 3,781,379, 2,888,943 and 4,091,049 to Labana et al.

These compositions, however, tend to provide high gloss finishes. Powdercoating compositions having a low gloss (matte) finish are becoming moredesirable for use in automotive interiors, and for wheel rims, bumpersand the like. A common way to reduce gloss is to incorporate a fillersuch as talc or silica into the composition. It is believed that thegloss is lowered by the formation of minute surface irregularities onthe coating. Another alternative is proposed in U.S. Pat. Nos. 5,436,311and 5,525,370 to Hoebeke et al. which proposes a low gloss compositionusing a high percentage of methyl methacrylate in the composition. Otherlow gloss compositions are proposed in U.S. Pat. Nos. 4,242,253 toYallourakis, and U.S. Pat. No. 5,491,202 to Umehara et al.

These low gloss compositions, however, are often not resistant toburnish. Namely, an article having low gloss will change to a high glossafter marring, scratching or rubbing. There is also a need to improveweatherability and durability.

Thus, there remains a need for a low gloss powder coating compositionthat has excellent resistance to degradation, namely is durable andweather resistant, and is burnish resistant.

SUMMARY OF THE INVENTION

To this end, the present invention provides a burnish resistant anddurable powder coating composition comprising a polyester resin havinghydroxyl and carboxyl groups and a glycidyl-containing acrylic copolymerwherein the polyester resin has a glass transition temperature(hereinafter “T_(g)”) less than about 55° C. and the glycidyl-containingacrylic copolymer has a number average molecular weight greater thanabout 8000.

In another embodiment, the present invention provides a powder coatingcomposition comprising about 20 to 90 percent by weight of a polyesterresin having hydroxyl and carboxyl groups, preferably having a T_(g) ofless than about 55° C., about 5 to 60 percent by weight of aglycidyl-containing acrylic copolymer and about 2 to 20 percent byweight of a blocked polyisocyanate derivative.

In still another embodiment, the present invention provides a method ofpreparing a burnish resistant coating on an article of manufacture. Themethod comprises applying to the article a powder coating compositioncomprising a polyester resin having hydroxyl and carboxyl groups and aglycidyl-containing acrylic copolymer and curing the coated article ofmanufacture preferably at a temperature of about 120° C. to 220° C.

DETAILED DESCRIPTION OF THE INVENTION

As summarized above, the powder coating comprises a polyester resinincluding carboxyl and hydroxyl functionality and a glycidyl-containingacrylic copolymer. The polyester resin includes carboxyl and hydroxylfunctionality, i.e., it is a “bi-functional” polyester. The amount ofpolyester, by weight is preferably from about 20 to 90 percent, andpreferably is about 25 to 75 percent by weight of the powder coatingcomposition.

With respect to the carboxyl portion of the polyester resin, thestarting acid and alcohol components thereof may be known conventionaldibasic and polybasic acids, and dihydric and polyhydric alcohols,respectively. Typical examples of the acid component includeterephthalic acid, isophthalic acid, phthalic acid, methylterephthalicacid, trimellitic acid, pyromellitic acid, adipic acid, sebacic acid,succinic acid, maleic acid, fumaric acid, tetrahydrophthalic acid,methyltetrahydrophthalic acid, hexahydrophthalic acid,methylhexahydrophthalic acid, and the anhydrides thereof. Typicalexamples of the alcohol component include ethylene glycol, propyleneglycol, 1,3-butanediol, 1,4-butanediol, 1,6-hexanediol, neopentylglycol, isopentyl glycol, bishydroxyethyl terephthalate, hydrogenatedbisphenol A, an ethylene oxide adduct of hydrogenated bisphenol A, apropylene oxide adduct of hydrogenated bisphenol A, trimethylolethane,trimethylolpropane, glycerol, pentaerythritol and2,2,4-trimethylpentane-1,3-diol. Monoepoxy compounds can also be used asone glycol component. The polyester resin preferably has an acid valueof about 5 to 100 mg KOH/g, and more preferably about 10 to 30 mg KOH/g.

With respect to the hydroxyl portion, the above alcohol components maybe used, namely ethylene glycol, propylene glycol, 1,3-butanediol,1,4-butanediol, 1,6-hexanediol, neopentyl glycol, isopentyl glycol,bishydroxyethyl terephthalate, hydrogenated bisphenol A, an ethyleneoxide adduct of hydrogenated bisphenol A, a propylene oxide adduct ofhydrogenated bisphenol A, trimethylolethane, trimethylolpropane,glycerol, pentaerythritol and 2,2,4-trimethylpentane-1,3-diol. Thepolyester resin has a hydroxyl value of about 5 to 100 mg KOH/g,preferably about 10 to 30 mg KOH/g. Preferably, the polyester resin is asofter resin, namely it has a T_(g) of less than about 55° C.

The blocked isocyanate derivative reacts With the hydroxyl group of thepolyester. The amount of blocked polyisocyanate derivative is preferablyfrom about 1 to 30 percent, and more preferably about 2 to 20 percent byweight of the total powder coating composition. Typical blockedisocyanates include products obtained by blocking the free isocyanategroups present in aliphatic, aromatic and alicyclic isocyanates such ashexamethylene diisocyanate, xylylene diisocyanate and isophoronediisocyanate or adducts of these. isocyanates with compounds havingactive hydrogen, with conventional blocking agents such as methanol,isopropanol, butanol, ethyl lactate and e-carprolactam. Examples of thecompounds having active hydrogen are various low-molecular weightcompounds such as ethylene glycol, propylene glycol, butylene glycol,trimethylopropane, glycerol, ethylenediamine and hexamethylenediamine,and various high-molecular-weight compounds such as polyols, polyethers,polyesters and polyamides.

Another class of blocked isocyanates derivatives are uretdionehardeners. Suitable uretdione hardeners are self-blocked uretdionegroups containing polyisocyanates. These uretdione group-containingorganic polyisocyanates can be prepared in accordance with well-knownmethods by dimerization of the corresponding organic polyisocyanate. SeeSaunders, J. H. and Frisch, K. C., Polyurethane: Chemistry andTechnology, Part I, pages 91-94 (1962) and U.S. Pat. No. 4,522,975 thedisclosure of which is incorporated by reference herein in its entirety.Any suitable organic polyisocyanate, or mixture of polyisocyanates,which is susceptible to dimerization can be used. Typical examplesinclude the following and mixtures thereof: 2,4-toluene diisocyanate;2,6-toluene diiosycanate; isomeric mixtures of 2,4- and 2,6-toluenediisocyanate; ethylene diisocyanate; propylene diisocyanate;methylene-bis (4-phenyl isocyanate); methylene-bis(4-cyclohexyl)isocyanate; xylene diisocyanate;3,3′-bitoluene-4-4′-diisocyanate; hexamethylene diisocyanate;naphthalene 1,5-diisocyanate; isophorone diisocyanate; and the like.Exemplary uretdione hardeners include Vestagon available from Huils,Germany or Crelan available from Bayer, Germany.

Another class of blocked isocyanate derivative is a 1,3,5 triazolavailable from Bayer as NB 650301 such as blocked isocyanate derivativeis used when a low temperature (e.g., less than about 150° C.) cure isdesired.

The glycidyl-containing acrylic copolymer is a polymer or copolymerobtained by using (β-methyl)glycidyl (meth)acrylate as a startingmaterial. It reacts with the carboxyl portion of the polyester resin.The term “copolymer” as used herein means a polymer of two or moredifferent monomers. The glycidyl-containing acrylic copolymer isobtained in a customary manner, for example, from about 10 to 100percent by weight, preferably about 15 to 60 percent by weight, of atleast one (β-methyl)glycidyl (meth)acrylate selected from the groupconsisting of glycidyl acrylate, glycidyl methacrylate, β-methylglycidyl acrylate and β-methyl glycidyl methacrylate. Especiallypreferred as the (β-methyl)glycidyl (meth)acrylate is glycidylmethacrylate either alone or in combination with β-methylglycidylmethacrylate. For the purpose of this invention, the glycidyl-containingacrylic copolymer preferably has a number average molecular weight ofgreater than about 8000 and less than about 20,000, and preferably 8,000to 16,000, and an epoxy equivalent of greater than about 425 and lessthan about 2000, and more preferably 525 to 650. Preferably about 5 to60 percent by weight of the total powder coating composition is theglycidyl-containing acrylic copolymer.

The remainder of the copolymer can be preferably about 0 to 80 percentby weight, preferably about 20 to 60 percent by weight, of another vinylmonomer. Typical examples of the other vinyl monomer which may be usedas a comonomer to be reacted with (β-methyl)glycidyl (meth)acrylate areone or more of the following: styrene, (meth)acrylic acid esters (e.g.,methyl methacrylate), furmaric acid diesters, acrylonitrile andacrylamide. In embodiments wherein weatherability is an issue, theso-called other vinyl monomer content is preferably reduced to less thanabout 30 percent by weight, particularly when the other vinyl monomer isstyrene.

The glycidyl-containing acrylic copolymer can also be modified with amonobasic acid, for example, by using about 5 to 70 percent by weight,preferably about 15 to 60 percent by weight, of at least one(β-methyl)glycidyl (meth)acrylate selected from glycidyl acrylate,glycidyl methacrylate, β-methyl glycidyl acrylate and β-methylglycidylmethacrylate, preferably about 0 to 60 percent by weight, morepreferably about 20 to 50 percent by weight, of another vinyl monomer,and preferably about 1 to 20 percent by weight, more preferably about 2to 10 percent by weight, of the monobasic acid. Examples of themonobasic acid are methyl acrylic acid, benzoic acid,p-tert-butylbenzoic acid, hydrogenated products of benzoic acids,caprylic acid, lauric acid, myristic acid, palmitic acid, stearic acid,arachic acid, behenic acid, lignoceric acid, and isomers thereof withbranched alryl groups. Polybasic acids which are esterified oretherified leaving only one carboxyl group may also be used as themonobasic acid. Modification of the glycidyl-containing acrylic polymerwith a monobasic acid can be carried out as described in U.S. Pat. No.4,499,239 to Murakami et al., the disclosure of which is incorporatedherein in its entirety by reference. Preferably, the glycidyl-containingacrylic copolymer has a T_(g) of less than about 60° C.

The copolymers are prepared using conventional techniques known to thoseskilled in the art. Preferably the monomers are prepared by free radicalpolymerization in the presence of an initiator. Suitable free radicalinitiators include di-tertiary butyl peroxide, benzoyl peroxide, deconylperoxide, azobisisobutyronitrile, and tertiary butyl peroctoate. Theinitiators are preferably present in amounts of from about 0.1 to 5percent.

A powder coating composition having improved performance can optionallybe obtained by the addition of a polyepoxy resin. The polyepoxy resinserves to improve the corrosion resistance and mechanical strength ofthe film, and denotes a resin preferably having at least 2, morepreferably 2 to 3, epoxy groups per molecule and an epoxy equivalentpreferably of 80 to 2,000, more preferably 100 to 1,000. Known epoxyresins may be used, and typical examples include diglycidyl ethers ofbisphenols and glycidyl ester ethers of hydroxybenzoic acid;polyglycidyl ethers of polyhydric alcohol, such as ethylene glycoldiglycidyl ether or trimethylol propane triglycidyl ether; glycidylesters of dibasic acids, such as diglycidyl terephthalate; alicyclicpolyepoxides such as diglycidyl ethers of hydrogenated bisphenols or(3,4-epoxy-6-methylcyclohexyl)methyl ester of3,4-epoxy-6-methyl-cyclohexanecarboxylic acid; and triglycidylisocyanurate. The amount of the polyepoxy resin is preferably about 0.5to 25 percent by weight, more preferably about 1 to 5 percent by weight.The polyepoxy resin suitably has a number average molecular weight ofpreferably 300 to 5,000, more preferably 500 to 3,000.

A flow control agent may be included in an amount preferably from about0.1 to 5 percent by weight percent of the total powder composition. Ingeneral, the flow control agent should be a polymer having a numberaverage molecular weight of at least 1000 and a T_(g) preferably about50° C. below the glass transition temperature of the compound copolymer.This flow control agent may be an acrylic polymer which may be preparedby polymerization of acrylate and methacrylate monomers in bulk or insuitable solvents using well known free radical initiators. The amountof initiator and polymerization conditions are chosen so that thepolymer has a molecular weight preferably above 1000, more preferablyabout 5000 and most preferably between about 6000 and about 20,000.Among the preferred acrylic polymers useful as flow control agents arepolylauryl acrylate, polybutyl acrylate, poly (2-ethylhexyl acrylate),polylauryl methacrylate and polyisodecyl methacrylate.

Although siloacrylate flow control agents are preferred, fluorinatedpolymers having a surface tension, at the baking temperature of thepowder, lower than that of the copolymer used in the mixture may beemployed. Preferred flow control agents, if the agent is a fluorinatedpolymer are esters of polyethyleneglycol or polypropylene glycol andfluorinated fatty acids. For example, an ester of polyethyleneglycol ofmolecular weight preferably over 2500 and perfluoro octanoic acid is auseful flow control agent. Polymeric siloxanes of molecular weight(M_(n)) preferably over 1,000, and more preferably 1,000 to 20,000, mayalso be useful as flow control agents, e.g., alkyl substituted silozanessuch as polydimethyl siloxanes, and halogenated siloxanes such as poly3,3,3-trifluoropropylnethyl silozane), poly(perfluorodimethyl siloxane,poly(pentafluorophenyl methyl siloxane), etc.

Since the powder coating compositions of the invention may be applied toarticles by electrostatic spray techniques, the composition may includea small weight percent of an antistatic agent. In particular, theantistatic agent is included in a range preferably from about 0.05 toabout 1.0 weight percent of the total powder composition. Suitableantistatic agents include, but are not limited to tetraalkyl ammoniumsalts as discussed previously and which also serve as catalysts. Othersuitable antistatic agents include: alryl poly (ethyleneoxy) phosphateor alkylauryl poly (ethyleneoxy) phosphates such as ethyl benzyl polyethyleneoxy) -phosphate, polyethyleneimine, poly (2-vinyl pyrolidone),pyridinum chloride, poly (vinyl pyridimium chloride), polyvinyl alcohol,and inorganic salts.

A plasticizer may be used in the powder coating compositions of thisinvention if desired. The types of plasticizers used very often includeadipates, phosphates, phthalates, secacates, polyesters derived fromadipic acid or azelaic acid, and epoxy or epoxidized plasticizers.Exemplary of the many plasticizers which may be employed are: dihexyladipate, diisooctyl adipate, dicyclohexyl adipate, triphenyl phosphate,tricresyl phosphate, tributyl phosphate, dibutylphthalate, ,dioctylphthalate, butyl octyl phthalate, dioctyl sebacate, butyl benzylsebacate, dibenzyl sebacate, butanediol-1,4diglycidyi ether andcellulose acetate butyrate.

Pigments may be included in an amount preferably from 0.1 to 50 percentby weight. Suitable pigments include titanium dioxide, iron oxides,organic dyestuffs carbon black, etc. Metallic pigments such as aluminummay be included to provide a metallic appearance.

The present invention provides a method of preparing a burnish resistantcoating on an article of manufacture which includes applying the abovepowder coating composition to the article. The powder coatingcomposition may be applied by known coating methods such aselectrostatic coating, electrostatic spray coating or fluidized coating.It may be applied to articles such as automotive exterior or interiors,appliance, lawn furniture, metal fixtures, and the like.

The coated film so prepared is baked or cured by a conventional methodat a temperature preferably of about 300° to 400° F. for at least 5minutes and preferably from 5 to 30 minutes to give a cured film havingsuperior appearance represented by superior smoothness, low gloss andoutstanding weatherability and is resistant to burnish.

EXAMPLE

The following Example illustrates the practice of the present invention.The Example should not be construed as limiting the invention toanything less than that which is disclosed. The process conditions areas follows:

Premixing: Henschel FM-10 Premixer, residence time=60 seconds per batch

Melt-Mix Extrusion: Werner & Pfleiderer ZSK-30, Zone 1:100 C, Zone 2: 80C, Screw Speed:250 RPM, Feed Rate: 60 lbs./hr.

Grinding: Brillrnann Retsch Mill, 15,000 RPM, 0.5 mm screen

Sieving: 140 mesh, Ro-Tap

Spray equipment: Nordson Versa-Spray 100, applied at −80kV

Raw Material Parts by Weight EM-33377 (1) 56.0 Bi-functional polyesterEM-27284 GMA acrylic (1) 31.0 Blocked IPDI hardener (2) 11.5Dicarboxylic Acid (3) 1.5 Flow Additive (4) 1.0 Benzoin (5) 0.5 CarbonBlack (6) 2.0 Barytes (7) 43.0 Catalyst (8) 0.2 Tin Catalyst (9) 0.35TOTAL 146.4 (1) REICHHOLD (2) HULS, Vestagon B-1530 (3) DUPONT,Dodecanedioc Acid, for enhanced film smoothness (4) MONSANTO, Modaflow200 Powder (5) GCA CHEMICALS, Uraflow B (6) COLUMBIAN CHEMICALS COMPANY,Raven 1255 Beads (7) HUBER, Huberbrite 1 (8) AIR PRODUCTS, Curezol C17imidazole for gloss stability (9) SYNTHRON SPECIALITY CHEMICALS, ActironDBT, urethane catalyst

Such a composition is applied to a substrate and cured for 10 minutes at204° C. The coated substrate is subjected to a burnishing event andchecked with a SEM photograph. The coated composition was resistant toblemish. Various properties of the composition are detailed in Table 1.

TABLE 1 Baking schedule 10 minutes at 400° F. Dry film thickness, mils2.5 Gloss, 60°/85° 4/8.5 PCI smoothness rating 6 (1-10, 10 = best)Pencil hardness 2H Gardner impact, in./lbs. F/R 160/160 Crosshatchadhesion 100% Solvent cure test, Full cure 100 MEK double rubs

The foregoing is illustrative of the present invention and is not to beconstrued as limiting thereof. Although a few exemplary embodiments ofthis invention have been described, those skilled in the art willreadily appreciate that many modifications are possible in the exemplaryembodiments without materially departing from the novel teachings andadvantages of this invention. Accordingly, all such modifications areintended to be included within the scope of this invention as defined inthe claims. Therefore, it is to be understood that the foregoing isillustrative of the present invention and is not to be construed aslimited to the specific embodiments disclosed, and that modifications tothe disclosed embodiments, as well as other embodiments, are intended tobe included within the scope of the appended claims. The invention isdefined by the following claims, with equivalents of the claims to beincluded therein.

That which is claimed is:
 1. A method of providing a burnish resistantcoating on an article of manufacture, the method comprising applying tothe article a powder coating composition comprising a polyester resinhaving hydroxyl and carboxyl groups, and having a T_(g) less than about55°, a glycidyl-containing acrylic copolymer, the copolymer having anumber average molecular weight greater than about 8000 and a blockedisocyanate derivative for reacting with the hydroxyl group of thepolyester resin, and curing the coated article of manufacture at atemperature of about 120° C. to 200° C.
 2. The method according to claim1, wherein said step of curing takes at least about 5 minutes.
 3. Themethod according to claim 1, wherein the blocked isocyanate derivativeis a uretdione group-containing organic polyisocyanate.
 4. The methodaccording to claim 1, wherein the polyester resin has a hydroxyl valueof about 10 to 30 mg KOH/g and an acid value of about 10 to 30 mg KOH/g.5. The method according to claim 1, wherein the glycidly-containingacrylic copolymer has a T_(g) of less than about 60° C.
 6. The methodaccording to claim 1, wherein the glycidyl-containing acrylic copolymeris glycidyl methacrylate.
 7. The method according to claim 1, whereinthe glycidyl-containing acrylic copolymer has an epoxy equivalent weightof greater than about
 425. 8. An article of manufacture preparedaccording to claim
 1. 9. A method according to claim 1, wherein thepowder coating composition comprises about 20 to 95 percent by weightpolyester resin having hydroxyl and carboxyl groups, about 5 to 60percent by weight of the glycidyl-containing acrylic copolymer, andabout 2 to 20 percent by weight of the blocked isocyanate derivative.